The invention relates to a metallic heat transfer tube, in particular for the evaporation of liquids from pure substances or mixtures on the outside of the tube.
Evaporation occurs in many areas of air conditioning and refrigeration engineering and in process and energy engineering. Shell and tube heat exchangers are often used in this type of engineering, in which exchangers liquids from pure substances or mixtures evaporate on the outside of the tube, and thereby cool off a brine or water on the inside of the tube. Such devices are identified as flooded evaporators.
By intensifying the heat transfer on the outside and the inside of the tube, it is possible to significantly reduce the size of the evaporator. This reduces the manufacturing costs of such devices. Furthermore the required filling capacity of refrigerant is reduced which, in the case of the current predominantly used HFCs, can amount to a significant portion of the entire cost of the system. Furthermore, the potential of danger can be reduced, in the case of toxic or flammable refrigerants, by a reduction of the filling capacity. The current common double enhanced tubes are more efficient by approximately a factor of three than plain tubes with the same diameter.
The present invention relates to structured tubes in which the heat transfer coefficient is intensified on the outside of the tube. Since the main portion of the heat transfer resistance is in this manner often shifted to the inside, the heat transfer coefficient must as a rule also be intensified on the inside. An increase of the heat transfer on the inside of the tube results usually in an increase of the tubeside pressure drop.
Heat transfer tubes for shell and tube heat exchangers have usually at least one structured area and one plain end and possibly plain center lands. The plain ends or center lands provide the limits of the structured areas. In order for the tube to be able to be installed without any problems into the shell and tube heat exchanger, the outer diameter of the structured areas may not be greater than the outer diameter of the plain ends and center lands.
In order to increase the heat transfer during the evaporation, the process of the nucleate boiling is intensified. It is known that the formation of bubbles starts at the nucleation sites. These nucleation sites are mostly small gas or vapor inclusions. Such nucleation sites can be produced already by roughening the surface. When the growing bubble has reached a certain size, it becomes detached from the surface. When the bubble becomes detached, the nucleation site is flooded with liquid and any included gas or vapor may also be displaced by the flooding liquid. The nucleation site is in this case inactivated. This can be avoided by a suitable design of the nucleation sites. It is here necessary to make the opening of the nucleation site smaller than the cavity lying below the opening.
It is known in the art to produce such structures on the base of integrally formed finned tubes. Integrally finned tubes are where the fins are formed out of the wall material of a plain tube. Various methods are known whereby the channels between adjacent fins are closed off in such a manner that connections between channel and surrounding area remain in the form of pores or slots. Since the opening of the pores or slots is less than the width of the channels, the channels represent suitably formed cavities, which favor the formation and stabilization of nucleation sites. Such essentially closed channels are created in particular by bending or tilting the fin (U.S. Pat. Nos. 3,696,861, 5,054,548), by splitting and flattening the fin (DE 2 758 526, U.S. Pat. No. 4,577,381), and by notching and flattening the fin (U.S. Pat. No. 4,660,630, EP 0 713 072, U.S. Pat. No. 4,216,826).
The strongest commercially available performance enhanced fin tubes for flooded evaporators have a fin structure with a fin density of 55 to 60 fins per inch on the outside of the tube (U.S. Pat. No. 5,669,441, U.S. Pat. No. 5,697,430, DE 197 57 526). This corresponds to a fin pitch of approximately 0.45 to 0.40 mm. It is principally possible to improve the performance of such tubes with a yet higher fin density or smaller fin pitch since this increases the nucleation site density. A smaller fin pitch requires automatically more delicate tools. However, more delicate tools are subjected to an increased danger of breakage and quicker wear. The presently available tools enable a safe manufacture of finned tubes with fin densities of 60 fins per inch at a maximum. Furthermore a decreasing fin pitch reduces the production speed of the tubes and consequently the manufacturing costs are increased.
It is known that performance-enhanced evaporation structures without changing the fin density can be produced on the outside of the tube by structuring the base of the groove between the fins. It is suggested in EP 0 222 100 to provide the base of the groove with indentations by means of a notching disk. The indentations at the base of the groove can have a V, trapezoidal or semicircular cross section and represent additional nucleation sites. However, the performance increases achievable by such structures in particular in the range of small heat fluxes no longer meet the demands of the market. The indentations represent furthermore a weakening of the core wall of the tube and result in a reduction of the mechanical stability of the tube.
A performance-enhanced heat transfer tube for the evaporation of liquids on the outside of the tube is to be provided during a uniform tubeside heat transfer and pressure drop and with the same manufacturing costs.
The purpose of the invention is met by providing in a heat transfer tube of the mentioned type, recesses which are arranged in the area of the base of the primary grooves helically extending between the fins, in such a manner that the recesses are designed in the form of re-entrant secondary grooves.
A re-entrant groove (see FIG. 1) exists when
in a sectional plane a not closed off field X can be found;
the field X can be closed off by means of a region AB;
a region PQ, with P, Q being part of a boundary of X, is found so that PQ is parallel to AB and the width of PQ is greater than the width of AB.
A re-entrant secondary groove offers for the formation and stabilization of nucleation sites clearly more favorable conditions than the simple indentations suggested in EP 0 222 100. The position of the re-entrant secondary grooves near the primary base of the groove is particularly advantageous for the evaporation process since the wall superheat is the greatest at the base of the groove and therefore the highest driving temperature difference for the bubble formation is available thereat.
After the forming of the fins material is according to the invention removed by suitable additional tools, from the area of the fin flanks toward the base of the groove so that not completely closed off cavities are created at the base of the groove, which cavities define the desired re-entrant secondary grooves. The cavities extend from the base of the primary groove toward the tip of the fins, whereby the cavities expand at a maximum up to 45% of the fin height H, typically up to 20% of the fin height H. The fin height H is thereby measured from the lowermost portion of the base of the groove, which was formed by the largest rolling disk, to the fin tip of the completely formed finned tube.